Pref panel introduction + perf list rewrite

Now the list of perf counters seen during the trace is global and not
copied at each refresh. It makes less allocations and especially behaves
far better to keep settings while moving in time.

Also, the perf preference panel, is now becoming a real preference panel
specific for each view. Right now it behaves weirdly on view others than
perf, but I'm working on it now.

Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>